A semiconductor laser is extremely small sized and responds to a drive current at a high speed, so is widely used as a light source of an optical disk device, optical communication apparatus, and laser printer.
As a rewritable optical disk, a phase change optical disk and a magneto-optic disk are widely known. The two are, however, different in the output of the laser beam emitted when recording, reproducing, and erasing. For example, by emitting a laser beam having a laser power at the time of reproduction lower than that at the time of recording, the information is read out without destroying the recorded bits.
An optical disk device emits converged light of the semiconductor laser to the optical disk and obtains an information signal and a servo signal from the optical disk, therefore the light reflected from the optical disk is returned to the semiconductor laser side as well to a certain extent. Scoop noise and mode hopping noise due to interference between this returned light and the emitted light occur and become causes of inducing C/N deterioration of the reproduction signal.
The high frequency superimposition method is known for reducing these. According to this high frequency superimposition method, in the reproduction mode, a high frequency current of 200 MHz to 600 MHz is superimposed on a DC bias current of the semiconductor laser.
In the recording mode, along with higher recording densities and higher speed transfers, use is being made of a modulation system as shown in FIG. 1A combining pulse width modulation and intensity modulation.
In this case, it is necessary to set the intensity of the laser beam emitted to a plurality of levels (four of P1 to P4 in the example of FIG. 1A). The shortest pulse width is also up to about several nsec.
In the example of FIG. 1A, the level set becomes P1>P2>P3>P4.
In the case of the control of these four values, P1 is the peak level and P4 is the bottom level. P2 and P3 are predetermined levels between the peak and the bottom (intermediate value level). For example, the erasing power for an optical disk capable of overwrite is set to the intermediate level P3. In this case, as shown in FIGS. 1A and 1B, P3 is set for portions forming spaces between recording marks RMK.
In a high density and high transfer rate optical disk, in order to obtain an error rate enabling recording and reproduction, it is required to sufficiently control the intensity of the laser beam in the different modes of recording, reproduction, and erasing.
However, a semiconductor laser changes remarkably in drive current and optical output characteristic depending on the temperature characteristic, so an APC (auto power control) circuit, i.e., a so-called semiconductor laser optical output control circuit, becomes necessary in order to set the optical output of the semiconductor laser at a desired intensity.
This APC circuit is generally roughly classified into two types according to its control system.
The first system monitors the optical output of the semiconductor laser by a light receiving element and forms an opto-electric negative feedback loop for constantly controlling the drive current of the semiconductor laser so that the light receiving current generated in this light receiving element (proportional to the optical output of the semiconductor laser) and a light emission instruction signal become equal.
The second system monitoring method is the sample/hold system monitoring the optical output of the semiconductor laser by the light receiving element when setting the power, forming an opto-electric negative feedback loop for controlling the drive current of the semiconductor laser so that the light receiving current generated in this light receiving element (proportional to the optical output of the semiconductor laser) and the light emission instruction signal become equal, holding the control value of this drive current even other than when setting the power, and modulating based on this held control value other than when setting this power.
The first system is desirable in each mode of the recording mode and the reproduction mode, but in power control in the recording mode, a plurality of power levels are set and the pulse width thereof is a small one of several nsec; therefore realization of the first system is difficult due to the limit on the operation speed of the light receiving element and the operation speed of the opto-electric negative feedback loop. Due to such a reason, an APC circuit performing control by the second system has been used in both of the modes.
FIG. 2 is a circuit diagram of an example of the configuration of an APC circuit (semiconductor laser optical output control circuit) employing the second system (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 9-63093 and Japanese Unexamined Patent Publication (Kokai) No. 9-115167).
This APC circuit has a laser diode (LD) 1 as the semiconductor laser to be controlled, laser power monitor use photo diode (PD) 2, current/voltage conversion circuit (I/V) 3, error amplifier 4, optical power setting voltage use switching circuit 5, optical power setting voltage sources 6-1, 6-2, . . . , 6-n, sample/hold circuits (S/H) 7-1, 7-2, . . . , 7-n, voltage/current conversion circuits (V/I) 8-1, 8-2, . . . , 8-n, switching circuit 9, current amplifier 10, control terminals T71, T72, . . . , T7n provided in the sample/hold circuits 7-1, 7-2, . . . , 7-n, and control terminal T9 of the switching circuit 9.
The LD 1 outputs the laser beam to be emitted to the optical disk. The PD 2 monitors the optical output of the LD 1.
The current/voltage conversion circuit 3 converts the output current of the PD 2 to a voltage which it supplies to the error amplifier 4. The error amplifier 4 detects a difference between the output voltage of the current/voltage conversion circuit 3 and the optical power setting voltage and outputs it as an error voltage to the sample/hold circuits 7-1, 7-2, . . . , 7-n. 
The optical power setting voltage use switching circuit 5 selects one of the optical power (laser power) setting voltages V61, V62, . . . , V6n by the optical power setting voltage sources 6-1, 6-2, . . . , 6-n and supplies the same to the error amplifier 4.
The sample/hold circuits 7-1, 7-2, . . . , 7-n sample control voltages output by the error amplifier 4 in accordance with the levels of sample gate signals input via the control terminals T71, T72, . . . , T7n, hold them, and supply the held voltages V1, V2, . . . , Vn to the voltage/current conversion circuits 8-1, 8-2, . . . , 8-n. 
The voltage/current conversion circuits 8-1, 8-2, . . . , 8-n convert the outputs of the sample/hold circuits 7-1, 7-2, . . . , 7-n from voltage signals to current signals I1, I2, . . . , In.
The switching circuit 9 switches the output currents I1, I2, . . . , In of the voltage/current conversion circuits 8-1, 8-2, . . . , 8-n in accordance with a switch timing signal SWT input via the control terminal T9 and supplies them to the current amplifier 11.
The current amplifier 11 amplifies the current signals as the outputs of the voltage/current conversion circuits 8-1, 8-2, . . . , 8-n switched by the switching circuit 9 and drives the LD 1 by the amplified current signal.
Next, an explanation will be given of the operation of the circuit of FIG. 2 with reference to FIGS. 3A to 3F.
For example, in the format FMT of a magneto-optic disk, as shown in FIG. 3A, before a data portion (DT) 114 serving as the recording region of each sector SCT, an address portion (ADR) 115 recording the address of the sector SCT therein and an ALPC (auto laser power control) portion 116 for setting optical power levels of the reproduction, erasing, and recording are provided.
In the address portion 115, the address information is read out in the reproduction mode. In the ALPC portion 116, the optical power levels are sequentially set in time series as shown in FIGS. 3B to 3E.
In the other section, currents I1 to In output by the voltage/current conversion circuits 8-1, 8-2, . . . , 8-n based on the control values V1 to Vn held by the sample/hold circuits 7-1, 7-2, . . . , 7-n are selected by the switching circuit 9, and the LD 1 is driven to emit light by the drive current ILD multiplied by K by the current amplifier 10. The optical output waveform set in this ALPC region 16 is shown in FIG. 3F.
When the sample gate signal SMGT input to the control terminal T71 of the sample/hold circuit 7-1 is set at the ‘High’ level, the output voltage of the current/voltage conversion circuit 3 and the optical power setting voltage V61 are compared by the error amplifier 4, the LD 1 is driven based on the control voltage V1 output by the error amplifier 4, and the laser power is set.
By setting this loop band at about several MHz, a pull-in operation is sufficiently carried out on the setting of the laser power by 1 μsec. The control voltage of this laser power is held at the sample/hold circuit 7-1 by making the sample gate signal SPGT to be given to the control terminal T71 of the sample/hold circuit 7-1 the ‘Low’ level. The other optical powers are similarly sequentially set.
Subsequently, in the data portion 114 of the sector, the current outputs of the voltage/current conversion circuits 8-1, 8-2, . . . , 8-n generated by these held control voltages are switched by the switching circuit 9. By this, the recording light emission waveform shown in FIG. 1A and the DC light emission of reproduction and erasure by the LD 1 become possible.
At the time of driving this LD 1, the APC circuit has become an open loop, so it is possible to easily generate the high speed pulse drive current ILD of the recording mode.
However, the laser power setting section of several μsec of this ALPC region is a considerably long period compared with the period of generation of the laser pulse at the time of recording, so there is an effect on the service life of the LD 1 (semiconductor laser). Further, in a semiconductor laser, by injecting a forward direction current to a PN junction to form an inversion distribution and changing the injection current, the inversion distribution changes. Along with this, the frequency of induction discharge changes and the intensity of the laser beam changes. This response is high speed, so modulation by the pulse current is possible, but there is the defect of the appearance of relaxation oscillation in the optical pulse.
A general electrical equivalent circuit of a semiconductor laser is represented by an RLC parallel circuit as shown in FIG. 4 and includes a DC resistor Rd, a parallel capacitor Cd, an inductor Lw of a lead, and a package capacitor Cp. The inductor Lw and parallel capacitor Cd form a low pass filter which governs the modulation band of the laser.
Due to the above, there are many factors for fluctuation in the pulse light emission characteristic of a semiconductor laser. Even among semiconductor lasers of the same type, there is considerable fluctuation due to the variation among lots.
When a step-like drive current is supplied to a semiconductor laser, there is a droop characteristic where the optical output changes along with a temperature rise of the semiconductor laser.
Due to this, a difference arises between the optical power set at the ALPC portion 116 and the pulse light emission power at the time of recording.
Further, as mentioned above, in the recording mode, along with higher recording densities and higher speed transfers, the modulation systems shown in FIGS. 1A and 1B are being adopted. In this case, it is necessary to set a plurality of intensities of the laser beam emitted.
In this case, the power is sequentially set in time series and a sufficient ALPC region is increasingly harder to secure.